1. Field of the Invention
Embodiments of the present invention generally relate to a method for plasma etching a molybdenum layer and, more specifically, to a method for etching a molybdenum layer during photomask fabrication.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A mask is typically a glass or a quartz substrate that has a layer of chromium on one side. The chromium layer is covered with an anti-reflective coating and a photosensitive resist. During a patterning process, the circuit design is written onto the mask by exposing portions of the resist to ultraviolet light, making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed to create a patterned. This pattern allows the exposed underlying chromium to be etched. The etch process removes the chromium and anti-reflective layers from the mask at locations where the resist was removed, i.e., the exposed chromium is removed.
Another mask utilized for patterning is known as a phase shift mask. The phase shift mask is similar to the mask described above, except that alternating adjacent areas of quartz regions exposed through the patterned chromium layer are covered with a layer of light-attenuating material. The thickness of the light-attenuating material is about equal to half the wavelength of light which will be utilized to transfer the circuit patterns to a substrate during fabrication. In one embodiment, the layer of light-attenuating material is about 50 nm and about 100 nm thick. It is contemplated that different thicknesses may be utilized. The attenuating material layer may be deposited by conventional methods known in the art, such as by chemical vapor deposition (CVD) techniques. Examples of suitable light-attenuating material include molybdenum silicide, molybdenum silicon (MoSi), molybdenum silicon oxynitride (MoSiXNYOZ), combinations thereof, or any other material suitable for shifting the phase of light passing therethrough.
As the light is shown through the phase shift mask to expose resist disposed on the substrate during circuit fabrication, the light impinging in the resist through one opening in the mask is 180 degrees out of phase relative to the light passing through the light-attenuating material covering the immediately adjacent opening. As a result, light that may be scattered at the edges of the mask opening is cancelled out by the 180 degree light out of phase scattering at the edge of the adjacent opening, causing a tighter distribution of light in a predefined region of the resist. The tighter distribution of light advantageously facilitates writing of features having smaller critical dimensions.
In one etch process, known as dry etching, reactive ion etching, or plasma etching, a plasma is used to enhance a chemical reaction and pattern chromium layer of the mask through a polymer resist. After the polymer resist is stripped, the patterned chromium layer is utilized as a mask to etch the light-attenuating material. Undesirably, conventional processes used to etch the light-attenuating material (e.g., molybdenum) often exhibit etch bias due to attack on sidewalls of the openings in the chromium layer utilized to pattern the light-attenuating material. As the openings are enlarged during the chromium etch process, the critical dimension of patterned chromium layer is not accurately transferred to the light-attenuating material. Thus, conventional molybdenum etch processes may not produce acceptable results for masks having critical dimensions less than about 5 μm. This results in non-uniformity of the etched features of the mask and correspondingly diminishes the ability to produce features for devices having small critical dimensions using the mask.
As the critical dimensions of mask continue to shrink, the importance of etch uniformity increases. Thus, the ability to accurately maintain critical dimensions during fabrication of a photomask is highly desirable.
Thus, there is a need for an improved molybdenum etch process suitable for photomask fabrication.